A manometer is a pressure-measuring instrument which balances fluid heads in a glass tube positioned opposite a scale. Pressure readings are taken by noting the registry of a maniscus or manisci with an indicated location or locations on the scale. In its simplest form, the manometer consists of a U-tube, one end of which is open to the atmosphere and the other end of which is open to the region where the pressure is to be measured. If the measured pressure is different from atmospheric pressure, the fluid with which the manometer tube is partially filled will stand higher in one leg of the tube than the other. The U-tube manometer indicates pressure as the vertical distance from the top of the fluid (i.e. the maniscus) in one leg to the top of the fluid (i.e. the maniscus) in the other leg. For ease of reading, the scale on this manometer is graduated from a center zero location toward each end of the scale and the distances from zero are added to obtain a pressure reading in inches of mercury, pounds per square foot or in other conventional units of pressure. If one leg of the tube is filled with indicating fluid, evacuated and sealed, the reading on a suitably calibrated scale will indicate the pressure at the open end of the tube in absolute pressure units, i.e. barometric pressure.
In a well-type manometer, one leg of the U-tube is in the form of a well which has much larger diameter than the other leg, i.e., the sight or indicating tube which is juxtaposed to the scale. As a result, a great change in column height in the indicating tube is accompanied by a minor change in the level of the well contents which change is referred to as the well-drop. This drop is corrected by the graduations on the scale. Thus the scale of a well-type manometer, although it may be graduated in terms such as inches of mercury, will not measure exact linear inches. Instead, the scale incorporates a correction factor which is identified as the well-drop factor and thus it reads out accurately directly in the units of measurement, e.g. inches of mercury. The well-type manometer offers many advantages in terms of convenience and versatility. For example, the design permits use of a direct-reading single indicating tube. This means that the pressure, vacuum, fluid flow or fluid level reading is taken directly from a single indicating column rather than by a measure of the difference between two column levels as in the U-tube manometer. By raising the well above the bottom of the indicating tube, the zero reading may be located at any specific point on the scale, say at eye level, so that readings above that point indicate pressure and readings below that point measure vacuum.
It is also a characteristic of a manometer that the vertical height of the fluid in the tube controls the range of the manometer. To obtain a more sensitive reading, then, the leg containing the fluid indicating column, i.e. the indicating tube, may be inclined and juxtaposed to an inclined scale, the remaining tube leg (or the well) remaining vertical. In this so-called inclined manometer, the inclined scale will expand the same range over a much longer scale so that there are more scale graduations per inch of vertical height which allows the scale to be read more easily and with more accuracy.
In practice, the above described different types of manometers are used in a variety of different settings. Some are supported on a table or other horizontal surface. Some are mounted to a wall or panel, being either back mounted, flush mounted or recessed. All manometers require plumbing and most are used with different accessories such as manifolds, valves, etc. depending upon the particular application.
The manometers available heretofore have tended to be relatively complex and costly instruments. This is because they are composed of numerous parts, many of which are machined parts made of hard, corrosion-resistant metals such as stainless steel in order to withstand the corrosive fluids to which they are exposed. Many have complicated scale adjust mechanisms involving flexible cables, rack and pinion mechanisms and the like. Some well-type instruments require an excessive amount of indicating fluid. One of the principal reasons for the high costs of prior manometers is due to the fact that even the instruments made by a given manufacturer do not have components which are interchangeable between the different manometer types, e.g. U-tube manometer, well-type manometer, inclined manometer, etc. Resultantly, not only are each manufacturer's parts costs for a complete line of manometers quite high, but also his inventory costs are excessive because he must maintain a complete supply of all the different spare parts for his entire line of instruments.
Some instruments also present a maintenance problem. For example, it is especially difficult to properly clean the prior well-type manometers, particularly their fluid pathways between the transparent indicating tubes and the wells. In the case of some manometers, many of their parts are welded together, making it difficult to gain access to the interiors of those instruments in order to repair and service them.
Also, as alluded to above, a manometer may have to be mounted in different ways depending upon the circumstances. In some cases, it must rest on a horizontal platform; in other situations, it must be mounted to a vertical wall or panel with a possibility of having to protrude from, be flush with or be recessed into the wall or panel surface. The prior instruments of this general type are not readily adapted to all of these mounting possibilities. Furthermore, prior manometers have tended to be rather large and bulky. Therefore, when several have to be mounted side by side in a row, they occupy a relatively large amount of wall or panel space.
Additionally, many conventional manometers have constructions and configurations which make it difficult to change their scale or to adjust the scale to zero. Some prior manometers are difficult to read. This is often due to the fact that their indicating tubes are retained in place by clips or brackets which obscure the scale or the tube maniscus. Sometimes, it is due to the fact that their scale graduations are located beside, rather than behind, the indicating tubes so that parallax errors are introduced into the readings from such instruments.
It would be desirable, therefore, if a single manometer design could be devised to address all of the aforesaid problems.